The use of Gas Diffusion Electrodes (GDEs) is known in several electrochemical processes. For example, hydrogen-oxygen fuel cells typically utilize the transformation of gaseous oxygen and hydrogen into liquid water at solid-phase, electrically-connected catalysts, like platinum metal.
At the present time, commercially available GDEs typically comprise of fused, porous layers of conductive particles (usually carbon particles) of different size. The outer-most layers typically contain particles of the smallest dimensions, fused together with smaller amounts of hydrophobic PTFE (polytetrafluoroethylene, or Teflon™) binder. The inner-most layers typically contain the largest particles. There may be multiple intermediate layers of intermediate particle size.
The intention of this gradation in particle size within GDEs, from largest in the center to smallest on the outer sides, is to create and control a three-phase solid-liquid-gas boundary within the electrode. This boundary should have the largest possible surface area. The creation of such a boundary is achieved, effectively, by controlling the average pore sizes between the particles, ensuring that the smallest pore sizes are at the edges and the largest are in the center. Since the pores are typically relatively hydrophobic (due to the PTFE binder), the small pore sizes at the edges (e.g. 30 microns pore size) act to hinder and limit the ingress of liquid water into the GDE. That is, water can penetrate only a relatively short distance into the GDE, where the electrochemically active surface area per unit volume, is largest. By contrast, the larger pores in the centre of the GDE (e.g. 150 microns pore size), allow for ready gas transmission at low pressure along the length of the GDE, with the gas then forming a three-way solid-liquid-gas boundary with the liquid water at the edges of the GDE, where the electrochemically active surface area per unit volume is the largest.
Layered porous electrode structures are presently the industry standard for:                (1) conventional free-standing GDEs (for example, of the type used in hydrogen-oxygen PEM fuel cells); and        (2) hybrid GDEs, where a GDE layer has been incorporated within an electrode, typically between a current collector and the gas zone.        
GDEs of this type often display significant technical problems during operation. These largely derive from the difficulty of creating a seamlessly homogeneous particulate bed, with uniform pore sizes and distributions, and uniform hydrophobicity (imparted by the hydrophobic PTFE binder within the GDE). Because of the resulting relative lack of uniformity in the GDE structure, the three-phase solid-liquid-gas boundary created within the GDE may be:                Unstable and fluctuating. The location of the boundary within the GDE may be subject to changing conditions during reaction which cause the boundary to constantly re-distribute itself to new locations within the GDE during operation.        Inhomogeneous. The boundary may be located at widely and unpredictably divergent depths within the GDE as one traverses the length of the GDE.        Inconsistent and ill-defined. At certain points within the GDE, there may be multiple and not a single solid-liquid-gas boundary.        Prone to failure. The boundary may fail at certain points within the GDE during operation, causing a halt to the desired chemical reaction. For example, a common failure mode is that the GDE becomes completely filled with the liquid phase, thereby destroying the three-phase boundary; this is known in the industry as “flooding”. Flooding is a particular problem in fuel cells, such as hydrogen-oxygen fuel cells, that require the feedstock gases to be humidified. Flooding may be caused by water ingress into the gas diffusion electrode via systematic, incremental percolation through the non-homogeneous pores of the electrode, or it may be caused by spontaneous condensation of the water vapour in the feedstock gas stream. In all cases, flooding induces a decline in the voltage output and power generation of such fuel cells.        
Problems of this type are not conducive to optimum operations and may result in uneven, low-yielding, incomplete or incorrect reactions, amongst others.
Conventional 3D Particulate Fixed-Bed Electrodes and GDEs
At the present time, 3D particulate fixed bed electrodes and gas diffusion electrodes (GDEs) are conventionally fabricated by mixing carbon black and PTFE powders and then compressing the solid mixture into a bulk, porous electrode.
The pore size of the resulting structure may be very roughly controlled by managing the particle size of the particulates used. However, it is difficult to achieve a uniform pore size throughout the electrode using this approach because particles, especially “sticky” particles like PTFE, often do not flow evenly and distribute themselves uniformly when compressed. A wide range of pore sizes are therefore typically obtained. It is, moreover, generally not possible to create structures with uniformly small pore sizes, such as 0.05 μm-0.5 μm in size.
The hydrophobicity of the structure is typically controlled by managing the relative quantity of PTFE incorporated into the structure. The PTFE holds the structure together and creates the required porosity. However, its quantity must be carefully controlled so as to impart the electrode with an appropriately intermediate hydrophobicity. An intermediate hydrophobicity is needed to ensure partial, but not complete water ingress. In the case of GDEs, this is needed to thereby create a solid-liquid-gas boundary within the carbon black matrix that makes up the electrode.
This method of constructing 3D particulate fixed bed electrodes and gas diffusion electrodes creates some significant practical problems when operating such electrodes in industrial electrochemical cells, particularly in electro-synthetic and electro-energy (e.g. fuel cell) applications. These problems include the formation of three-way solid-liquid-gas boundaries that are: ill-defined, inconsistent, unstable, fluctuating, inhomogeneous, and prone to failures like flooding.
Problems of this type largely arise from the intrinsic lack of control in the fabrication process, which attempts to create all of the inherent properties of the electrode—including porosity, hydrophobicity, and conductivity—in a single step. Moreover, the fabrication method seeks to simultaneously optimise all of these properties within a single structure. This is often not practically possible since the properties are inter-related, meaning that optimising one may degrade another.
Despite these drawbacks, the approach of combining particulate carbon black and PTFE into a compressed or sintered fixed bed remains the standard method of fabricating GDEs for industrial electrochemistry. This approach is used to fabricate, for example, free-standing GDEs of the type used in hydrogen-oxygen PEM fuel cells. Even where only a GDE component is required within an electrode, the standard method of fabricating that GDE component is to form it as a compressed, porous layer of particulate carbon black and PTFE.
For the above and other reasons, the conventional method of making GDEs and the properties of conventional GDEs are open to improvement.
FIG. 1 (prior art) depicts in a schematic form, a conventional 3D particulate fixed bed electrode or a gas diffusion electrode (GDE) 110, as widely used in industry at present.
In a conventional 3D particulate fixed bed electrode or GDE 110, a conductive element (e.g. carbon particles) is typically combined (using compression/sintering) with a non-conductive, hydrophobic element (e.g. polytetrafluoroethylene (PTFE) Teflon™ particles) and catalyst into a single, fixed-bed structure 110. The fixed-bed structure 110 has intermediate hydrophobicity, good but not the best available conductivity, and a pore structure that is non-uniform and poorly defined over a single region 113. When the 3D particulate fixed bed electrode or GDE 110 is then contacted on one side by a liquid electrolyte and on the other side by a gaseous substance, these physical features bring about the formation of an irregularly-distributed three-phase solid-liquid-gas boundary within the body of the electrode 110, below its outer surface 112 and within single region 113, as illustrated in the magnified view presented in FIG. 1. At the three-phase boundary, electrically connected catalyst (solid phase) is in simultaneous contact with the reactants (in either the liquid or the gas phase) and the products (in the other one of the liquid or gas phase). The solid-liquid-gas boundary within the GDE 110 therefore provides a boundary at which electrochemical liquid-to-gas or gas-to-liquid reactions may be facilitated by, for example, the application of a particular electrical voltage. The macroscopic width of the three-phase solid-liquid-gas boundary is comparable or similar in dimension to the width of the conventional GDE. The thickness of the three-phase solid-liquid-gas boundary in a conventional GDE is typically in the range of from 0.4 mm to 0.8 mm in fuel cell GDEs up to higher thicknesses, such as several millimeters, in industrial electrochemical GDEs.
The phenomenon of flooding described above, is often caused by water ingress into the gas diffusion electrode when the water is subject to any sort of external pressure. For example, in an industrial electrolytic cell of 1 meter height, the water at the bottom of the cell is pressurised at 0.1 bar due to the hydraulic head of water. If a GDE were used at this depth, the GDE would typically be immediately flooded by water ingress because modern-day GDEs have very low “wetting pressures” (also known as the “water entry pressure”), that are typically less than 0.1 bar (although GDEs with wetting pressures of 0.2 bar have recently been reported in WO2013037902). GDEs are, additionally, relatively expensive.
This is a particular problem in industrial electrochemical cells in which it is highly beneficial to apply a gas such as oxygen or hydrogen to the counter electrode, via the use of a GDE at that electrode.
Depolarization
In many industrial electrochemical processes, the counter electrode is not productive in that the counter electrode does not produce a desired and valuable product. Instead, the counter electrode produces a waste product that must be disposed of, typically at some cost. In such cases, one may “depolarize” the counter-electrode by introducing a gas such as oxygen or hydrogen to the surface of that electrode, to thereby change the half reaction that occurs at the electrode and reduce the theoretical overall cell voltage by about 1.2 V.
For example, in the traditional chlor-alkali process, which is one of the most widely used industrial electro-synthetic processes in the world, chlorine is generated at the anode from acidified 25% NaCl solution, while hydrogen is generated at the cathode from strongly caustic solution (typically 32% NaOH). The hydrogen is not wanted and must be disposed of. The electrode half-reactions are as follows:
At the Anode:2Cl− → Cl2 + 2e−E0ox = −1.36 VAt the Cathode:2H2O + 2e− → H2 + 2OH−E0red = −0.83 VE0cell = −2.19 V
The negative sign for the Ecell indicates that the overall reaction is not thermodynamically favoured and needs to be driven by the application of an external electrical voltage. A positive sign for the Ecell would indicate that the overall reaction is spontaneous and generates a voltage and an electrical current. That is, it would indicate that the cell will act as a fuel cell.
As the cathode in a traditional chlor-alkali process is not productive, it can be depolarized by the addition of oxygen gas to thereby substantially decrease the overall cell voltage. The oxygen gas is most effectively introduced by using a gas diffusion electrode (GDE) at the cathode and passing the oxygen through the GDE into the system. The electrode half-reactions will then be:
At the Anode:2 Cl− → Cl2 + 2e−E0ox = −1.36 VAt the Cathode:O2 + 2 H2O + 4e− → 4OH−E0red = 0.40 VE0cell = −0.96 V
As can be seen, oxygen depolarization of the cathode in this manner reduces the cell voltage by more than half, and thereby effects a substantial improvement in the energy consumption involved in the manufacture of chlorine.
At the present time, GDEs are incorporated in only a small number of industrial applications for the purposes of depolarizing counter electrodes. This mainly involves the production of chlorine from hydrochloric acid by the companies Industrie De Nora S.p.A., Bayer AG, and ThyssenKrupp Uhde AG.
Most industrial electrochemical processes that could benefit from electrode depolarization do not presently make use of electrode depolarization. This is largely because of the expense and practical difficulties of using conventional GDEs.
For example, chlor-alkali cells are generally more than 1 meter in height. If a conventional GDE was used to depolarize the anode, which comprises one wall of the cell, the GDE would flood at the base of the GDE causing the highly caustic 32% NaOH solution to leak from the electrolyte chamber in the cell. This would occur because current-day, conventional GDEs typically flood at 0.1 bar liquid pressures. It is, consequently, not tenable to use current-day conventional GDEs in such cells.
Attempts have been made to overcome this problem. For example, WO2003035939 teaches the use of a somewhat cumbersome “gas pocket” design of electrode which allows for the introduction of oxygen at the cathode without leaking of the caustic electrolyte from the electrolyte chamber. WO2003042430 similarly seeks to overcome the problem by the use of a “percolator-type” cathode, which efficiently breaks hydraulic heads in liquid chambers but has the undesired properties of being expensive and adding ohmic drops to the cell construction.
More recently, WO2013037902, assigned to the electrode specialist company Industrie De Nora S.p.A., describes a novel fabrication technique to realise a GDE capable of withstanding 0.2 bar liquid pressure, which exceeds the 0.1 bar pressure produced by water under a 1 meter hydraulic head. The GDE described in WO2013037902 is, nevertheless, expensive and leaves little margin for error in that only a 0.1 bar overpressure will be enough to cause the highly caustic electrolyte to leak from the electrolyte chamber of the cell. Any defects in the GDE—no matter how tiny-will result in or create a risk of caustic leakage. Moreover, special manifolding is required on the cell to balance pressures.
Similar or comparable situations or problems pertain in numerous other industrial electrochemical processes that may benefit from the use of gas depolarized GDEs, if they were practically viable. These include the electrochemical manufacture of: (a) hydrogen peroxide, (b) fuels from CO2, (c) ozone, (d) caustic (without chlorine), (e) potassium permanganate, (f) chlorate, (g) perchlorate, (h) fluorine, (i) bromine, (j) persulfate, and others. Electrometallurgical applications, such as metal electrowinning, could also benefit from the energy savings associated with anode depolarization; metal electro-deposition occurs at the cathode side of such cells, while oxygen is evolved at the anode. If oxygen evolution was replaced by hydrogen oxidation on a suitable gas diffusion anode, this would generate substantial energy savings. However, the mechanical characteristics of conventional GDEs make them unsuitable for delimiting narrow-gap chambers, thereby restricting their application in the undivided electrolysis cells that are widely used in electrometallurgical processes. Moreover, conventional GDEs would leak under the hydraulic head of electrolytic solutions commonly used in industrial size electrolysers. Several industrial electrochemical processes in the pulp and paper industry may also benefit from the use of gas depolarized GDEs, including: (a) “black liquor” electrolysis, (b) “Tall Oil recovery” and (c) chloride removal electrolysis. Flooding of GDEs after the build-up of even very mild liquid pressures is, furthermore, a particular and well-recognized problem in fuel cells, such as hydrogen-oxygen fuel cells.
In summary, a need exists for a Gas Diffusion Electrode that can be gas depolarized and utilised in electro-synthetic, electrochemical, fuel and/or electro-energy cells or devices. Preferably, the Gas Diffusion Electrode should be relatively inexpensive, robust and/or mechanically strong, and have a relatively high wetting pressure. There is a need for such Gas Diffusion Electrodes that can, consequently, be readily, generally and/or beneficially deployed as gas diffusion and gas depolarized electrodes in a variety of industrial electrochemical, fuel, electro-energy and/or electro-synthetic processes, cells and/or devices.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.